114 



ECHINODERMA. OPHIUROIDEA 



the oral surface of the disc ; they are in the form of slits 

 in the interbrachial areas, usually two in each, situated 

 near the bases of the arms ; occasionally there are four. 

 The aboral surface of the disc (fig. 45 B) is, in most cases, 

 covered with numerous small plates ; but usually there is 



Fig. 45. A. Ophiura, Recent. Oral surface of disc and part of the 

 arms, b, buccal plates ; a, genital slits ; v, ventral plates of arms. 

 B. Ophioglypha, Recent. Aboral surface, r, radial plates ; I, lateral 



plates of arms ; d, dorsal plates of arms. 



xli. 



at the bases of the arms on each side a large plate, the 

 radial plate (r) ; there may also be a large plate at the 

 centre of the disc surrounded by one or more circles of five 

 plates. On this surface the arms end abruptly at the 

 margin of the disc. 



Protaster. Disc well marked, with fairly large scales. Arms 

 five, long, tapering, without ventral plates ; ambulacral ossicles 

 boot-shaped, alternating, not united. Ordovician and Silurian. 

 Ex. P. sedgivicki, Silurian. 



Lapworthura. Disc circular, well marked. Arms flexible, 

 broad, at first of uniform width, afterwards tapering ; ambulacral 

 ossicles opposite, fused, their distal and proximal margins parallel, 

 with plain articular surfaces ; ventral arm-plates and buccal plates 

 absent. Madreporic plate dorsal, large. Ludlow Beds. Ex. L. 

 miltoni. 



